A data communications network is a collection of hardware and software that uses communications channels to share data and information between users and devices connected to the network. Millions of users worldwide connect to a data communications network on a daily basis for accessing information, shopping, recreation, and conducting business. Examples of data communications networks include wired networks such as local area networks at a home or place of business and wide area networks such as the Internet, as well as local and wide area wireless networks such as Bluetooth and 802.11 networks.
Users may access a data communications network with a variety of network devices, which are electronic devices configured with a network access system, such as personal and portable computers, electronic organizers, personal digital assistants (“PDAs”), entertainment systems, stereo systems, video game units, household appliances, or other embedded electronic devices. All of these devices currently require a local source of power to operate when connected to a data communications network.
With the adoption of the recently established 802.3af Ethernet standard, commonly known as the “Power over Ethernet” (“PoE”) standard, it is expected that powered networks will become more widespread. Powered networks carry data and provide DC power to a whole new class of “powered devices” (“PDs”), all of which will not require additional AC wiring or external power sources to connect to the powered network.
The power is provided by a device called a “Power Sourcing Equipment” (“PSE”) that is typically placed in an Ethernet hub, switch, router, or other network equipment. PDs may include digital IP telephones, wireless network access points, PDA or portable computer docking stations, cell phone chargers, HVAC thermostats, or almost any network device that can run from the DC power provided by the powered network.
The power is typically applied as a common mode voltage difference between two powered wire pairs, by powering the center taps of the isolation transformers used to couple the differential data signals to the wires. As the powered network data lines are transformer-isolated at each end of a wire, the potential difference between the transmit pair and the receive pair has no effect on the data transceivers at either end of the wire.
PSEs contain a detection mechanism to detect the presence of a PD at a network port prior to sending power to it and to prevent sending power and causing permanent damage to devices that are non-compliant with the PoE standard, i.e., devices that are not designed to receive power from the powered network such as the currently-available personal and portable computers. The detection mechanism involves probing the network cable for a “PD signature” before applying voltage to the wire.
Once the power is on, the PSE must keep it on as long as the PD presents a valid “power maintenance signature”. The power maintenance signature specified in the PoE standard, for example, consists of a minimum DC current draw of at least 10 mA and an AC impedance at or lower than 27 kΩ at all frequencies from DC to 500 Hz. The. PSE can opt to monitor either or both components of this power maintenance signature to determine if the PD is still present at the network port. If the PSE determines that a PD has removed its power maintenance signature indicating that the PD is no longer connected to the network cable, the PSE removes power from the network to prevent power from being delivered to a non-compliant device that may eventually be inserted into the same network port.
Accordingly, the PoE standard specifies two methods for probing a network connection for a power maintenance signature and removing power from the network if the power maintenance signature is no longer present: (1) a “DC disconnect” method, that involves monitoring a minimum DC current draw of at least 10 mA; and (2) an “AC disconnect” method, that involves monitoring the AC impedance of the network port.
The DC disconnect method involves the use of a current monitoring circuit in the PSE for monitoring the current flow through the network connection by measuring a return current DC signal. If the current monitoring circuit determines that the return current DC signal dropped below the minimum allowed current level of 10 mA, thereby signifying that a PD has been unplugged from the network port or that the PD has removed its power maintenance signature, the PSE removes the power supplied to the network.
Alternatively, the AC disconnect method involves the use of an AC disconnect sensing circuit for imposing a time varying signal that is capacitively coupled to the connection on top of the voltage that is powering the PD and using this time-varying signal to measure the AC impedance of the network port. As specified by the PoE standard, a PD is considered present at the network port if the AC impedance of the network port is equal to or lower than 27 kΩ. Conversely, a PD may be considered unplugged from the network port if the AC impedance of the network port is greater than a threshold set above 27 kΩ. The PoE standard further specifies that a PD must be considered unplugged from the network if the AC impedance of the network port is equal to or greater than 1980 kΩ. The PSE removes the power supplied to the network when a PD has been unplugged from the network, i.e., when the power maintenance signature is no longer present. The AC disconnect sensing circuit may be located either at the endpoint of the network link or at the center of the link, commonly referred to as “midspan”.
A problem relating to the AC disconnect threshold specified by the PoE standard is that the capacitance of the network cable due to combinations of long cable runs, connectors, patch panels, outlet boxes and other equipment used to carry data may be high enough for the pair to pair impedance between the PSE and the PD to indicate that a PD is present at the network port when, in fact, it has been removed. That is, the AC disconnect sensing circuit may measure an AC impedance that is due to the pair to pair reactive impedance of the cable instead of the AC impedance indicative of the presence or absence of a PD. As a result, the AC disconnect sensing circuit may inadvertently maintain the power delivery to the network regardless of whether a PD is present or not, with the potential to permanently damage a non-compliant device that may eventually be inserted into the network-port later in the absence of a PD.
Additionally, as the PSE supplies power to the network at a DC voltage, it necessarily has a low output impedance that would prevent an AC disconnect sensing circuit from measuring anything other than the PSE's output impedance. To mitigate this problem, a coupling diode is inserted in series with the PSE to enable the AC disconnect sensing circuit to disregard the PSE's output impedance. However, the PSE output impedance may be disregarded only when the AC disconnect sensing circuit measures the AC impedance of the network port when the coupling diode is reverse biased. Otherwise, the AC disconnect sensing circuit would measure the impedance of the coupling diode instead of the impedance of the network port. And even when the AC impedance is measured when the coupling diode is reverse biased, the PSE bypass capacitance required by the PoE standard to meet its specifications for stability, ripple, and load regulation may still contribute to the AC impedance of the connection. As a result, the AC disconnect sensing circuit may measure an impedance that is not due solely to the presence of a PD in the network port.
Another constraint is that the PoE standard requires the AC disconnect sensing circuit to operate at less than 500 Hz, thus slewing the network port voltage to less than 100 V/ms and 4.4 V peak-to-peak. To meet this low slew rate and low frequency signal requirement, the AC coupling capacitor must be close to 1 μF and be able to handle 60 V of DC bias.
In view of the foregoing, it would be desirable to provide circuits and methods for detecting the presence of a powered device in a powered network connection and removing power from the powered network connection when no powered device is present.
It further would be desirable to provide circuits and methods for measuring the AC impedance of a powered network connection and determining whether to remove power from the connection based on the measured AC impedance.
It also would be desirable to provide circuits and methods for measuring the AC impedance of a powered network connection and distinguishing between the impedance of the connection due to the network cable, PSE bypass capacitance, and coupling diode and the impedance of the network port indicative of the presence or absence of a PD.